Dekameter Radar Observations of Ocean Wave Growth and Decay

Abstract

Immediately following the passage of polar fronts that produced strong winds blowing away from Galveston Island and into the Gulf of Mexico, we used scattered LORAN-A radio waves, together with observations by wave stags, to measure the growth of 80 m waves as a function of angle to the wind, and the decay of the same wavelengths going against the wind. The LORAN signals at 2 MHz are in Bragg resonance with 80 m (7 s) ocean waves offshore, and were used to map the strength of both approaching and receding 80 m waves as a function of range and azimuth using Doppler velocity to determine wave directions. We found that 1) these waves grow as the cosine of the angle to the wind; 2) growth rates averaged over all angles are comparable to those previously published, 3) growth rates measured as a function of time by the wave staff were numerically the same as growth measured as a function of distance by the radar; 4) growth appears to be inhibited for waves having frequencies near the peak frequency of preexisting waves going against the wind, but apparently not at higher frequencies; 5) attenuation of waves going against the wind is very weak compared with growth rates under the same conditions; and 6) ?1% of wave energy incident on a very gently sloping natural beach is reflected back to sea.